Telomerase is an enzyme known to have the function of repairing the length of telomere which is shortened by each cell division (Greider C. W. and Blackburn E. H., (1987) Cell, 51, 887-898; Morin G. B. (1989) Cell, 59, 521-529).
Most cancer cells show telomerase activity (Kim N. W. et al., (1994) Science, 206, 2011-2015), a finding strongly suggesting that telomerase takes part in maintaining the infinite proliferation of cancer cells.
The measurement of telomerase activity, therefore, is important for diagnosis of cancer, and a substance inhibiting telomerase activity can be expected to serve as an anticancer drug causing few adverse reactions to normal cells (Counter C. M. et al., (1989) EMBO J., 11, 1921-1929; Counter C. M. et al., (1994) Proc. Natl. Acad. Sci. USA, 91, 2900-2904; Chadenneau C. et al., (1995) Cancer Res., 55, 2533-2536; Hiyama E. et al., (˜1995) Nature Med., 1, 249-255; Shay J. W. et al., (1995) Mol. Cell. Biol., 15, 425-432).
One of conventional methods for measuring telomerase activity is to measure telomerase enzyme activity.
According to this method, a cell extract needs to be prepared beforehand, with enzyme activity being maintained. Then, a telomere elongation reaction (telomerase reaction) is performed. The elongated telomere is measured for the amount of the resulting DNA, directly or after amplification by polymerase chain reaction (PCR). This method was not a convenient, effective method for measuring telomerase activity.
Another method for measuring telomerase activity is to measure the expression of a telomerase gene. Such a method requires the identification of the gene correlated to telomerase activity.
As one of such genes, a gene encoding a protein having a reverse transcriptase motif with a molecular weight of about 130 kDa was recently isolated from mRNA derived from a human testis or a cancer cell (Meyerson M., et al., (1997) Cell, 90, 785-795; Nakamura T. M. et al., (1997) Science, 277, 955-959).
Since the expression of this gene showed high correlation with telomerase activity, this gene has been speculated to code for a human telomerase catalytic subunit.
However, much is yet unknown about telomerase.
For example, it is still unknown whether a plurality of genes having a reverse transcriptase motif exists, and whether a gene, or a plurality of genes, showing telomerase activity exist. It is also unknown whether the telomerase activity of a normal germ cell and that of a cancer cell always come from the same genetic product, and whether the telomerase activities of all cancer cells can be explained by a single gene. Solutions to these questions are desperately wanted in the fields of cancer diagnosis and treatment focusing on telomerase.